1. Field of the Invention
The present general inventive concept relates to an apparatus to detect a sync signal, a VSB receiver using the same, and a method thereof, and more particularly to an apparatus to detect a sync signal using a plurality of partial noncoherent correlators, a VSB receiver using the same, and a method thereof.
2. Description of the Related Art
In a receiver for receiving data transmitted in a vestigial side band (VSB) modulation method, a frequency offset and a phase noise generated by a tuner or a radio frequency (RF) oscillator used in the receiver should be minimized in order to demodulate the data. This process is called “carrier recovery.” A digital broadcasting system employing the VSB modulation method based on the standard of the Advanced Television System Committee (ATSC), which is an American digital television standard, uses a pilot signal existing in a transmitted signal for carrier synchronization. The pilot signal refers to a signal that is loaded on a carrier during a data transmission in order to accurately recover the carrier.
A process of generating the same clock that is used in a transmitter in order to receive accurate data in the receiver is called “symbol timing recovery.”
FIGS. 1A and 1B each illustrate an operation of detecting a sync signal in a conventional VSB receiver. FIG. 1A is a block diagram illustrating a sync detection unit that uses a correlation of an “I” signal, and FIG. 1B is a block diagram illustrating a sync detection unit that uses a noncoherent correlation.
Referring to FIG. 1A, the sync detection unit that uses the correlation of the “I” signal includes a correlator 10, a maximum value detection unit 30, and a position detection unit 50.
The correlator 10 calculates a correlation value between the “I” signal that is a received real signal and a reference signal.
The maximum value detection unit 30 detects a maximum value among the correlation values calculated by the correlator 10.
The position detection unit 50 detects a position where the maximum value is detected, as a start point where effective data exists.
Accordingly, sync detection using the correlation of the “I” signal can be calculated using Equation (1),
                              ∑                      k            =            i                                i            +            N            -            1                          ⁢                              r            ⁡                          (              k              )                                ⁢                      p            ⁡                          (              k              )                                                          (        1        )            where r(k) represents the received real signal that corresponds to the “I” signal, p(k) represents the reference signal, N represents a number of samples of the received signal, and k is an index.
Accordingly, after the correlation value between the “I” signal being the received real signal and the reference signal is calculated as in Equation (1), a value “i” that corresponds to the maximum correlation value can be detected using Equation (1), thereby obtaining the sync signal.
Referring to FIG. 1B, the sync detection unit that uses the noncoherent correlation includes a first correlator 21, a second correlator 23, a first squarer 25, a second squarer 27, an adder 29, a maximum value detection unit 30, and a position detection unit 50.
The first and second correlators 21 and 23 calculate a first correlation value between the “I” signal of the received signal and the reference signal, and a second correlation value between a “Q” signal of the received signal and the reference signal, respectively.
The first and second squarers 25 and 27 square the correlation value calculated by the first correlator 21 and the correlation value calculated by the second correlator 23, respectively.
The adder 29 adds the value calculated by the first squarer 25 to the value calculated by the second squarer 27.
The maximum value detection unit 30 detects a maximum one of output values of the adder 29, and the position detection unit 50 detects a position of a signal having the maximum value detected by the maximum value detection unit 30.
Accordingly, in the sync detection method that uses the noncoherent correlation, the correlation can be calculated using Equation (2),
                                          [                                          ∑                                  k                  =                  i                                                  i                  +                  N                  -                  1                                            ⁢                                                r                  ⁡                                      (                    k                    )                                                  ⁢                                  p                  ⁡                                      (                    k                    )                                                                        ]                    2                +                              [                                          ∑                                  k                  =                  i                                                  i                  +                  N                  -                  1                                            ⁢                                                                    r                    ′                                    ⁡                                      (                    k                    )                                                  ⁢                                  p                  ⁡                                      (                    k                    )                                                                        ]                    2                                    (        2        )            where r(k) represents the received real signal, r′(k) represents a received imaginary signal, p(k) represents the reference signal, N represents the number of samples of the received signal, and k is an index.
Accordingly, as in Equation (2), the correlation value between the “I” signal and the reference signal is calculated and is then squared, and the correlation value between the “Q” signal and the reference signal is calculated and is then squared. Further, a value “i” that corresponds to a maximum one of the added values of each of square values can be detected, thereby obtaining the sync signal. In other words, it can be determined that the effective data is positioned after a point of “i”.
However, in the sync detection method described with reference to FIG. 1A, that uses the correlation of the “I” signal as the received real signal, the sync signal is detected using only the received real signal and therefore, influence by the phase noise cannot be removed. In other words, in the sync detection method using the correlation of the received real signal (“I” signal), the sync signal is detected using only the “I” signal, and not using the “Q” signal and therefore, there is a drawback in that the sync signal with the phase noise is detected, thereby making it impossible to detect an accurate sync signal. Specifically, in the case where a phase is twisted at ±90°, error between a data frame and a maximum value of a coherent correlation can be also one symbol.
The sync detection method described with reference to FIG. 1B that determines the noncoherent correlation, uses all of the “I” signal as the real signal and the “Q” signal as the imaginary signal, and is generally used when the phase noise exists in the received signal. The sync detection method that uses the noncoherent correlation is based on an aspect that the imaginary signal has no relation between a magnitude and a phase, thereby removing the influence of the phase noise from the received signal and detecting the sync signal.
In particular, the correlation value of the “I” signal and the correlation value of the “Q” signal are squared by the first squarer 21 and the second squarer 23, respectively, and then the respective squared values are added together by the adder 29, so that phase portions are removed from the “I” signal and the “Q” signal having a phase difference of 90° therebetween, thereby not reflecting the phase noise in sync detection.
However, in the case where the phase noise and the carrier frequency offset both exist in the received signal, the sync signal cannot be detected accurately even in the sync detection method using the noncoherent correlation. In other words, there is a drawback in that since the correlation value is varied by the carrier frequency offset, the sync signal cannot be accurately detected. In the case where the carrier frequency offset exists in the received signal, the correlation value can be expressed as in Equation (3),
                                                                                          [                                                            ∑                                              k                        =                        i                                                                    i                        +                        N                        -                        1                                                              ⁢                                                                  r                        ⁡                                                  (                          k                          )                                                                    ⁢                                              p                        ⁡                                                  (                          k                          )                                                                                                      ]                                2                            +                            ⁢                                                [                                                            ∑                                              k                        =                        i                                                                    i                        +                        N                        -                        1                                                              ⁢                                                                                            r                          ′                                                ⁡                                                  (                          k                          )                                                                    ⁢                                              p                        ⁡                                                  (                          k                          )                                                                                                      ]                                2                                                                                        =                            ⁢                                                                    [                                                                  ∑                                                  k                          =                          i                                                                          i                          +                          N                          -                          1                                                                    ⁢                                                                        p                          ⁡                                                      (                            k                            )                                                                          ⁢                                                                                                  ⁢                        cos                        ⁢                                                                                                  ⁢                                                  (                                                      2                            ⁢                                                                                                                  ⁢                            π                            ⁢                                                                                                                  ⁢                                                          f                              0                                                        ⁢                            t                                                    )                                                ⁢                                                  p                          ⁡                                                      (                            k                            )                                                                                                                ]                                    2                                +                                                                                                      ⁢                                                [                                                            ∑                                              k                        =                        i                                                                    i                        +                        N                        -                        1                                                              ⁢                                                                  p                        ⁡                                                  (                          k                          )                                                                    ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                                              (                                                  2                          ⁢                                                                                                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      f                            0                                                    ⁢                          t                                                )                                            ⁢                                              p                        ⁡                                                  (                          k                          )                                                                                                      ]                                2                                                                                        =                            ⁢                                                                    [                                                                  ∑                                                  k                          =                          i                                                                          i                          +                          N                          -                          1                                                                    ⁢                                                                                          ⁢                                              cos                        ⁢                                                                                                  ⁢                                                  (                                                      2                            ⁢                                                                                                                  ⁢                            π                            ⁢                                                                                                                  ⁢                                                          f                              0                                                        ⁢                            t                                                    )                                                                                      ]                                    2                                +                                                      [                                                                  ∑                                                  k                          =                          i                                                                          i                          +                          N                          -                          1                                                                    ⁢                                                                                          ⁢                                              sin                        ⁢                                                                                                  ⁢                                                  (                                                      2                            ⁢                                                                                                                  ⁢                            π                            ⁢                                                                                                                  ⁢                                                          f                              0                                                        ⁢                            t                                                    )                                                                                      ]                                    2                                                                                        (        3        )            where r(k) represents the received real signal, r′(k) represents the received imaginary signal, p(k) represents the reference signal, N represents the number of samples of the received signal, k is an index, and f0 represents the carrier frequency offset.
As illustrated in Equation (3), when the carrier frequency offset (f0) exists, even in the case where frame synchronization is accurate, the correlation value is varied by the carrier frequency offset (f0). In the case where the frame synchronization is accurate, a sum of the square value of the correlation value of the “I” signal and the square value of the correlation value of the “Q” signal should be “1.” However, the sum of the respective square values does not become “1” due to the carrier frequency offset (f0) and therefore, the sync signal cannot be accurately detected.